Isolation of conditional lethal mutator mutants of Escherichia coli by localized mutagenesis

By using localized mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine, we isolated 39 temperature-sensitive growth mutants that exhibited high mutability when the bacteria were grown at the permissive temperature. Two of the mutations, dnaQ186 and dnaQ231, were shown to be new alleles of the dnaQ gene by genetic mapping and complementation tests with the dnaQ49 mutation previously isolated. They shared common properties with the dnaQ49 strain, but their mutator activity was not temperature dependent. The dnaQ mutants exhibited increased sensitivity to inhibitors of DNA gyrase and to DNA intercalating and alkylating agents.